In a mobile communication system including a plurality of cells, when a mobile station (User Equipment: UE) moves from one cell to another cell, the mobile station continues communication by switching the cell. Such switching of the cell is called “Mobility control.” More specifically, it is called a “cell reselection,” or “handover.”
When received power or received quality of a signal from a neighboring cell becomes greater than receiving power or received quality of a signal from a serving cell, a mobile station executes a cell reselection or handover to the neighboring cell.
A cell reselection is a process, with which a mobile station being in a waiting state (i.e., an idle mode) transfers from a serving cell to a neighboring cell. Handover is a process, with which a mobile station being in a communication state (i.e., a connected mode) transfers from a serving cell to a neighboring cell.
Namely, a mobile station measures received power or received quality of signals from a serving cell and a neighboring cell, respectively, regardless of whether the mobile station is in the waiting state or the mobile station is in a state, in which the mobile station transmits data to and receives data from the serving cell.
Here, the received power of the signal from the neighboring cell or the serving cell is, for example, “received power (Reference Signal Received Power: RSRP) of a downlink reference signal (Reference Signal: RS),” which is transmitted from the neighboring cell or the serving cell. The received quality is, for example, “received quality (Reference Signal Received Quality: RSRQ) which is a ratio between RSRP and total received power of a down link (Received Signal Strength Indicator: RSSI)” (cf. Non-Patent Document 1, for example).
Here, a case is explained in which a mobile station is not able to accurately measure received power or received quality of a serving cell or a neighboring cell. For example, when the mobile station determines the received power or the received quality of the serving cell or the neighboring cell to be greater than its actual value, it is possible that a cell reselection or handover is not correctly executed. In this case, it is possible that a suitable mobile communication service is not provided to the mobile station in the serving cell which continues serving or in the neighboring cell to be transferred to. Accordingly, quality of service for the user is degraded.
Namely, it can be said that more accurate measurement is required for a cell reselection or handover.
Further, during a waiting state, a mobile station determines whether it is served based on received power or received quality of a serving cell, in addition to measurement for a cell reselection. For example, a mobile station compares received power or received quality of a signal from a serving cell with a predetermined threshold value. When the received power or the received quality of the signal from the serving cell is less than or equal to the predetermined value, the mobile station determines that it is out of a serving area. For other cases, the mobile station determines that it is within the serving area.
The predetermined threshold value may be reported by signaling from a network, i.e., a radio base station. For example, in a Long Term Evolution (LTE) scheme, the predetermined threshold value is called as “Qrxlevmin” or “Qqualmin.”
Here, when the mobile station determines that it is out of the coverage area, despite that it is within the coverage area, it is possible that a mobile communication service is not provided to the mobile station in an area in which the mobile communication service can be provided. Because of this, the quality of service is degraded for the user.
Conversely, when the mobile station determines that it is within the serving area, despite that it is out of the serving area, namely, when the mobile station determines that it is within the serving area, even though radio quality is so poor that communication may not be executed, it is possible that a user may not recognize that the user himself/herself is out of the serving area. For the service quality, it is not preferable to determine that it is within the serving area, despite that it is out of the serving area.
Namely, for the determination of whether it is within the serving area or out of the serving area, it is necessary to accurately measure the received power or the received quality. In other words, when a measured result of the received power or the received quality is not accurate, degradation of the service quality is caused.
Measurement of received power or received quality in a mobile station is not particularly specified. Namely, a measurement bandwidth or a measurement interval (a measurement time period) for measuring received power or received quality depends on implementation.
An LTE mobile communication system is explained. An LTE mobile communication system is a mobile communication system that is a successor of W-CDMA (Wideband Code Division Multiple Access) and HSDPA (High Speed Downlink Packet Access).
In LTE, downlink reference signals to be measured by a mobile station are arranged while they are distributed both in a frequency axis direction and in a time axis direction (cf. Non-Patent Document 2, for example). The mobile station can suitably set a measurement bandwidth or a measurement interval for measuring the received power or the received quality, provided that it is within a range in which certain accuracy is satisfied.
Next, the LTE mobile communication system is explained.
FIG. 1 shows an example of system bandwidths.
In the LTE mobile communication system, a variable system bandwidth from 6 resource blocks (Resource Block: RB) to 100 resource blocks may be used, depending on a capability of an evolved Node B (eNodeB) and user equipment. For example, a system bandwidth of 6 resource blocks may be used, a system bandwidth of 50 resource blocks may be used, or a system bandwidth of 100 resource blocks may be used. A resource block is a single transmission unit in a frequency direction. A frequency bandwidth of one resource block is 180 kHz (cf. Non-Patent Documents 3 and 4, for example). In LTE, the system bandwidth is also called “Channel Bandwidth.”